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Are the base of all food chains in the sea, except for the chemosynthetic communities associated with hydrothermal vents. The few marine flowering plants that grow along the coastal fringes include mangroves, eel and turtle grasses, and salt marsh plants. Most marine plants are varieties of algae. In coastal habitats large seaweeds, the browns, reds, and greens, grow attached to the seabed, especially to submerged rocks. Only one species, sargasso weed, is free floating.

In the open ocean marine plants are components of the plankton, known as phytoplankton. The largest phytoplankton are 1–2 millimetres across, whereas the smallest are a thousand times smaller. Animals can sieve large cells out of the water but not those that are less than 0.005 mm across. So the grazers of these tiny cells either have to be extremely small themselves, or use sticky webs of mucus to entrap them. The phytoplankton includes several types of algae, including diatoms and dinoflagellates. Diatoms have cells enclosed within pill-box glassy walls, etched so finely that they are used to assess the quality of microscope lenses. Sinking diatoms are a major source of organic matter to the deep-living communities. Dinoflagellates can swim using long whiplike flagellae. Blooms of some form highly toxic red tides; many produce the phosphorescence seen in surf at night. The smallest cells are known as picoplankton. These are mostly tiny flagellates, and in tropical seas are often responsible for 80% of the photosynthesis.